A Wonk Room analysis finds that there are six key Senate races in which a strong vote for climate action runs a serious risk of being replaced by a global-warming denier. Of the 37 U.S. Senate races this fall, 36 involve Republican candidates who are global-warming deniers or oppose climate action (Vermont’s Len Britton is a possible exception). Hard-right Tea Party challenger Christine O’Donnell knocked out climate activist Mike Castle on Tuesday, leaving a GOP slate of conspiracy theorists and know-nothings angling for the United States’ highest legislative body. Here are the pivotal climate races this November (along with the latest 538.com estimated likelihood of a Republican pickup):

Colorado: Michael Bennet vs. Ken Buck

Sen. Michael Bennet (D), appointed to fill the seat left by Secretary of the Interior Ken Salazar, is attempting to win his first election against Weld County District Attorney Ken Buck. Bennet supports climate legislation, while Buck has said, “While I think the earth is warming, I don’t think that man-made causes are the primary factor.”

Washington: Patty Murray vs. Dino Rossi

Climate activist Sen. Patty Murray (D), first elected in 1992, is in a tight race with real-estate businessman Dino Rossi, who believes “there’s still a lot of debate going on this, we see it out there and there’s going to be a big debate going on for the next two, three years.”

The rest of the races are unlikely to significantly change the Senate climate policy math. Some races involve long-shot deniers against safe incumbents, others involve the reelection of standing climate opponents, and some mean the replacement of a pro-pollution Democrat with an even more pro-pollution Republican — such as Blanche Lincoln’s and Evan Bayh’s seats.

The greatest determining factor for climate policy in the U.S. Senate is majority control, which determines what legislation moves through committee and onto the floor, and what issues are investigated. An Inhofe-led Environment and Public Works Committee would mean hearings about the “global warming hoax” instead of markups of climate legislation. Even if Democrats retain control of the Senate, which 538’s Nate Silver estimates is a 79 percent likelihood, the committee ratios will be adjusted to reflect lost seats, making it more likely that voting blocs of Republicans and anti-climate Democrats could overwhelm progressives on key votes.